Field of the Invention
The present invention relates to a structure of an optical waveguide device which is manufactured on a substrate, and in particular, relates to the design of an electrode and a waveguide of an optical modulation device.
Description of the Related Art
Currently, the amount of information used in optical communication has been increasing. In order to respond to such an increase in the amount of information, a response such as an increase in a signal speed and an increase in the number of channels due to wavelength multiplexing communication has progressed in an optical communication network such as a backbone, a metro, or an access. According to this, a system necessary for optical communication is complicated, and thus problems such as an increase in a device size, an increase in the cost, and an increase in power consumption occur.
In addition, even in a data center which has been recently increased, a response to an increase in the amount of information is required, as with the network. In the related art, an electrical signal was mainly transmitted through a metal cable in communication between computers in the data center. However, recently, optical communication using an optical fiber has been used due to demand for further increase in the speed and of further decreasing power consumption. Furthermore, even in each level such as inside a board and inside the CPU of the computer, there is a problem in optical communication introduction.
As means for solving such problems in the optical communication network and for realizing optical communication introduction with respect to new fields, a light integrated circuit of a planar light circuit (PLC) formed of quartz (silica), and a high speed operation device of a ferroelectric such as lithium niobate (LN), and the like have been used from the related art. Recently, in addition to this, an optical device using a high refractive index material such as silicon, InP, and GaAs has been also attracting attention. Research and development of a planar optical waveguide device (an optical device) have progressed in various places (for example, refer to P. Dong, L. Chen, and Y-kai Chen, “High-speed low-voltagesingle-drive push-pull silicon Mach-Zehnder modulators”, Optics Express, 2012, Vol. 20, Issue 6, p. 6163-6169 (hereinafter referred to as “NPL 1”), P. Dong, C. Xie, L. Chen, L. L. Buhl, and Y.-K. Chen, “112-Gb/s Monolithic PDM-QPSK Modulator in Silicon”, European Conference and Exhibition on Optical Communication, 2012, Th.3.B.1 (hereinafter referred to as “NPL 2”), T.-Y Liow, K.-W. Ang, Q. Fang, J.-F. Song, Y-Z. Xiong, M.-B. Yu, G-Q. Lo, and D.-L. Kwong, “Silicon Modulators and Germanium Photodetectors on SOI: Monolithic Integration, Compatibility, and Performance Optimization”, IEEE Journal of Selected Topics in Quantum Electronics, 2010, Vol. 16, p. 307-315 (hereinafter referred to as “NPL 3”), and T. Tsuchizawa, K. Yamada, H. Fukuda, T. Watanabe, J. Takahashi, M. Takahashi, T. Shoji, E. Tamechika, S. Itabashi, and H. Morita, “Microphotonics devices based on silicon microfabrication technology”, IEEE Journal of Selected Topics in Quantum Electronics, 2005, Vol. 11, p. 232-240 (hereinafter referred to as “NPL 4”)).
A wavelength of light in a medium is inversely proportionate to a refractive index of the medium, and thus, in silicon of which a refractive index is high as approximately 3.5, a size such as a core width of an optical waveguide decreases. In addition, by using a medium such as silica of which a refractive index is considerably different from that of silicon of the core in a clad, an optical waveguide having high confinement is obtained. As characteristics of such an optical waveguide, a decrease in a radius of curvature is included. From these reasons, it is possible to decrease the size of an optical device using the optical waveguide, and thus it is possible to further decrease the size if the required functions of the optical device are same, and it is possible to realize various functions if the required size of the optical device are same.
In addition, by using silicon which is an electrically controllable semiconductor material, it is possible to realize a device having variable properties such as an optical modulator.
Furthermore, the optical device using silicon has a common element with a semiconductor device such as a CPU or a memory of the related art in a technology and a device used for a manufacturing process, and thus it is possible to reduce the cost due to quantity production. In addition, the optical device using the silicon can also integrate the optical device on the same substrate as that of a semiconductor device of related art. In this case, an electrical signal transmitted through metal wiring of the related art is replaced with an optical signal transmitted through the optical waveguide, and thus it is possible to increase the speed of additional instruments and to reduce power consumption.
In such circumstances, the optical modulator is one of major devices in the optical communication of converting the electrical signal into the optical signal, and has been studied by various institutes as an element for realizing an optical integrated device, as with other devices (for example, refer to NPL 1 to NPL 3).
In a Mach-Zehnder optical modulator using a semiconductor, a traveling-wave electrode is used in which the phase of the optical signal and the phase of the electrical signal are matched, and a voltage is applied from the outside of a substrate. Termination is performed on the outside of the substrate, but the termination may be performed on the substrate by arranging a resistor, or by forming a resistor in the process. On the other hand, in at least the input of the voltage, the voltage is usually applied from the outside of the substrate. In an LN modulator of the related art, as illustrated in FIG. 2, optical waveguides 113 and 115 before and after a Mach-Zehnder interferometer 120 are arranged on a straight line from a light incidence unit 112 to a light launching unit 114, and an earth electrode 130G, and an input unit 131 and an output unit 132 of a signal electrode 130S are formed on the side of the Mach-Zehnder interferometer 120. An electrical signal input from the outside is applied to an electrode on the substrate through the input unit 131 on the side of the substrate connected to the outside by wire bonding or the like. However, it is known that a traveling-wave electrode on a silicon substrate has a large propagation loss in the electrical signal, and when the electrode in a portion other than the phase modulation unit 124 is elongated, a loss due to a high frequency is remarkable in which modulation efficiency decreases due to a loss in power of the electrical signal, and thus a decrease in a modulation bandwidth is caused.
The present invention has been made in view of the above-described situation, and an object of the invention is to provide an optical modulation device which can shorten a distance from a phase modulation unit of an electrode to a device end portion.
In addition, the present invention is to provide an optical modulation device which can shorten a distance from the phase modulation unit of the electrode to the device end portion and is able to realize a further decrease in the size by decreasing a substrate in a longitudinal direction.